Modeling flat surface shapes on a cylinder
A possible solution is to use a boolean modifier

Starting with your round shape:

 

Create a cube and rotate it in the angle you need for the flat surface:



Then place the cube where you need the cut:



On the pelvis object add a boolean modifier, usnig difference mode ans selecting the cube as object:



To view the results just move the cube to a different layer or make it invisible (H) and unrenderable (CtrlH 



The advantage of this method is that you can still adjust the angle and placement by moving the cube, and when you are done you can apply the modifier.

The disadvantage is that you'll end up with some n-gons



Other way to do this is with the knife tool, in wireframe mode choose a side view for the cut and press K along with the Zkey to greate a straight cut:



Then you'll have to do some cleanup to get rid of unwanted vertices.